The present invention relates generally to rectification in converters and, more particularly, to circuits and methods for synchronous rectification in resonant converters.
Resonant converters are circuits often used to convert a direct current (DC) voltage in a DC power to an increased or decreased DC voltage, making the resonant converters DC-to-DC converters. The conversion process may include inverting the DC power to an alternating current (AC) power, increasing or decreasing the voltage of the AC power, and converting the AC power back to the DC power with the DC voltage increased or decreased. The process of converting a power from AC to DC is often called rectification. Resonant converters have different classifications based on how the resonant converters are constructed (for example, series loading versus parallel loading) and controlled (for example, fixed frequency versus variable frequency, zero-current switching (ZCS) versus zero-voltage switching (ZVS), and continuous resonance versus discontinuous resonance). One such classification of resonant converters is the clamped series resonant converter (CSRC).
Referring now to FIG. 1, a power conversion circuit 10 including a CSRC 12 known to those skilled in the art is shown. An AC power source 14 providing an AC power is coupled to CSRC 12 via three circuit components. AC power source 14 is coupled in series with an electromagnetic interference (EMI) filter 16 to remove EMI from the AC power provided by AC power source 14. EMI filter 16 is coupled to a four-diode full-wave bridge rectifier (diode bridge) 18 to rectify the AC power from AC power source 14 to a DC power after the AC power has passed through EMI filter 16. Diode bridge 18 outputs the DC power so a DC link voltage VDCL is provided to a DC link 20. Diode bridge 18 is coupled in parallel with a capacitor bank 22, which includes one or more capacitors, to smooth the DC link voltage, with the smoothed voltage then being provided to CSRC 12. CSRC 12 acts as a DC-to-DC converter in this case. As will be described further below, CSRC 12 inverts the DC power from diode bridge 18 to an AC power, increases the voltage of the AC power, and converts the AC power back to the DC power with an increased DC voltage. The output of CSRC 12 is coupled to a filter capacitor 24, which is coupled in parallel with a load 26.
CSRC 12 includes a half-bridge circuit 28 with two switches 30, 32. Switches 30, 32 may be any appropriate electronic switches such as metal-oxide-semiconductor field-effect transistors (MOSFETs), for example. Switches 30, 32 are coupled in series with each other and in parallel with capacitor bank 22. Switch 30 includes a switch body 34 in parallel with a body diode 36 and a parasitic junction capacitor 38, and switch 32 is coupled in parallel with a switch body 40 in parallel with a body diode 42 and a parasitic junction capacitor 44. Switches 30, 32 are controlled by a control system 46, which may include any appropriate electronic controller, such as an integrated circuit, for example. Control system 46 controls switches 30, 32 to invert the DC power from diode bridge 18 to an AC power. Switches 30, 32 are also coupled in parallel with two clamping diodes 48, 50, which are coupled in series with each other.
CSRC 12 also includes a resonant circuit 52, which includes a resonant capacitor 54 coupled in parallel with clamping diode 50, a resonant inductor 56 coupled to a node 58 between switches 30, 32, and a magnetizing inductor 60 coupled in series with resonant capacitor 54 and resonant inductor 56. A voltage transformer 62 configured as a single transformer hybrid coil is coupled in parallel with magnetizing inductor 60. While magnetizing inductor 60 is shown as a discrete circuit element coupled in parallel with voltage transformer 62, it is well known in the art that magnetizing inductor 60 is intrinsic to voltage transformer 62 and represents the magnetization of the core of voltage transformer 62. Voltage transformer 62 includes a primary coil 64 coupled in parallel with magnetizing inductor 60 and a secondary coil 66 isolated from primary coil 64. Secondary coil 66 includes a first coil section 68 and a second coil section 70 so secondary coil 66 includes three outputs 72, 74, and 76. A power input into primary coil 64 will appear at outputs 72, 74, but because output 76 is bridged between first coil section 68 and second coil section 70, no power will appear at output 76. Voltage transformer 62 transforms the AC power provided by switches 30, 32 and input into primary coil 64 into an AC power with a scaled voltage output at outputs 72, 74.
Outputs 72, 74 are coupled to a full-wave-rectifier circuit 78, which includes a rectification diode 80 coupled between output 72 and a node 82 and a rectification diode 84 coupled between output 74 and node 80. The anode of rectification diode 80 is coupled to output 72, and the anode of rectification diode 84 is coupled to output 74. The cathodes of rectification diodes 80, 84 are coupled to node 82. The full-wave-rectifier circuit 78 rectifies the AC power with the increased voltage output at outputs 72, 74 because of the configuration of rectification diodes 80, 84. Since rectification diodes 80, 84 are only activated by a positive voltage drop, the voltage across load 26 is always positive. Filter capacitor 24 and load 26 are each coupled between node 82 and output 76.
While rectification diodes 80, 84 of full-wave-rectifier circuit 78 are effective to provide a DC power to load 26, there are efficiency problems associated with diode rectification. For example, a forward voltage drop across a diode becomes significant when the output voltage drops, reducing a converter's efficiency. Therefore, many skilled in the art have turned to synchronous rectification, which includes using rectification switches in place of rectification diodes. However, drive circuits for rectification switches are often quite complicated and large, increasing the cost and energy consumption of using rectification switches. Therefore, the efficiency gained by replacing rectification diodes with rectification switches is lost, which leads to many skilled in the art choosing to keep the rectification diodes in their resonant converters.
It would therefore be desirable to provide a drive circuit for synchronous rectification in resonant converters that is both simplified and smaller to reduce cost and energy consumption.